Genetic diversity of Tn1546-like elements in clinical isolates of vancomycin-resistant enterococci

We have investigated the genetic diversity of Tn1546 among 17 vancomycin-resistant enterococci (VRE) isolates of Enterococcus faecium. Most of these multidrug-resistant strains harboured plasmids of 2 kb to >300 kb in size. The vancomycin resistance marker vanA was located on both the plasmid and the chromosomal DNA. VRE isolates 18 and 22 failed to amplify the orf1-IR(R) and orf2-IR(R) but contained the orf1 and orf2. VRE3 failed to amplify the orf1, orf2, vanR and vanS, but still yielded a larger than expected (4.4 kb vs. 2.3 kb) vanSH amplicon. VRE9, 10, 21 and 22 also yielded larger (5.5 kb) vanSH amplicons; all others yielded 4.0 kb vanSH amplicons. Sequence analysis of the vanSH amplicons from VRE9, 10, 21 and 22 revealed the presence of IS1251 between the vanS and vanH genes in these isolates. The observed vanSH amplicon from VRE3 contained orf31, orf30 and orf29 of the plasmid pRUM followed by the vanHAXYZ region of Tn1546. Translocation of Tn1546 to a pRUM-like plasmid in VRE3 resulted in the loss of its orf1, orf2, vanR and vanS elements and a loss of the orf32 of pRUM, leading to a unique structural arrangement of vanA elements that is hitherto unknown.